What Is a Thermowell and What Type Do You Need?

If you’ve ever ordered a thermocouple, you have probably ordered a thermowell alongside it. What you may be less clear on is what the thermowell is doing, why it matters, and whether you are specifying the right one for your application. This article covers the basics.

What Is a Thermowell?

A thermowell is a machined protective sleeve that houses a thermocouple or RTD. The thermowell acts as a barrier between the sensor and the process. It protects the sensor from direct exposure to conditions that would cause fast deterioration of thermocouple components while still allowing for accurate monitoring of process temperature. 

Common processes that require thermowells are any process that involves heating corrosive chemicals, high pressure chambers, and extreme temperatures.

BDC supplies thermowells through Pyromation, one of North America’s leading manufacturers of thermowell and temperature sensing products. BDC is one of a small number of authorized Pyromation representatives in the country, which means direct access to Pyromation’s full product line, custom configurations, and engineering support.

Do You Always Need One?

Not all applications require thermowells. In low-pressure, clean environments, a thermocouple or RTD can go directly into the process. Measuring air temperature in a duct, for example, is an example where direct insertion works fine.

The thermowell becomes necessary when process conditions threaten the sensor: corrosive fluids, elevated pressure, high heat, and aggressive flow conditions that would shorten sensor life or cause premature failure. If your process checks any of those boxes, thermowells are necessary for your process. 

How Thermowells Work: The Four Connection Types

The pressure and temperature requirements of the application determine the right connection type. In some cases, sanitation requirements may also determine the type of thermowell you need. 

• Threaded thermowells are the most common in standard industrial applications. They carry the lowest pressure rating of the four types and are the easiest to install and remove.
• Flanged thermowells are used when a threaded connection falls short. They use a bolt-up connection rated to a specific pressure class. Ratings of 150 lb and 300 lb are the most common in general industrial work, with higher classes available for more demanding applications. 
• Weld-in thermowells offer the highest pressure and temperature ratings. Once installed, the well is permanent. The sensor can still be removed and replaced, but the well itself requires cutting out if it ever needs to come out. These see the most use in chemical and high-pressure environments.
• Sanitary thermowells are designed for food and dairy applications where the well must be removable for regular cleaning. A polished interior finish distinguishes the sanitary type from a standard weld-in.

In some industrial settings, particularly stacks and high-temperature furnace environments, a simpler pipe-type well is used in place of a machined thermowell. This involves a capped piece of pipe with a welded bushing rather than a machined component. It is a lower-cost approach suited for higher-temperature applications where measurement precision is a secondary concern.

Pyromation manufactures all four connection types. We carry their full thermowell line and can source any configuration, standard or custom, for your application.

How Far Should a Thermowell Protrude Into a Pipe?

Insertion depth, called the U dimension on thermowell drawings, is the most critical specification. It determines how far the well extends into the process and how far the sensor reaches into the pipe or vessel.

The internal bore of the thermowell is sized to match the sheath length of the thermocouple or RTD going inside it. A spring-loaded connection holds the sensor tip in firm contact with the bottom of the well, which is essential for accurate readings. 

How far a thermowell should protrude into a pipe depends on pipe diameter and process conditions, as well as where in the flow path the temperature needs to be measured. As a general principle, the tip should reach the center third of the pipe for a representative reading. For anything outside a direct replacement, it is worth confirming the insertion depth with a supplier who knows the application.

Wells range from under two inches to 36 inches in length depending on the process.

Choosing the Right Material

Common thermowell materials include carbon steel, 304 stainless, 316 stainless, 446 stainless, and Hastelloy C. The choice comes down to two factors: process temperature range and chemical exposure.

A practical starting point is matching the thermowell material to what the rest of the pipe or vessel is built from. Customers replacing an existing well often specify the same material already in use on that line. A thermowell placed in a temperature that’s consistently higher than it is rated for will degrade faster than expected.

In natural gas combustion applications, thermowell lifespan is typically long because the process does little damage to the well. In chemical processing environments, replacement frequency is considerably higher.

The Response Time Tradeoff

A thermowell places roughly a quarter inch of material between the process and the sensor tip. Temperature has to travel through that material before reaching the sensor, which introduces a slight lag in readings.

For most industrial temperature monitoring, this is a necessary tradeoff. The protection the well provides outweighs the small delay. For applications where rapid temperature changes need to be detected in near real time, it is worth discussing the response time impact with a specialist before finalizing the spec.

What to Have Ready When You Order

If you are replacing an existing thermowell, the fastest path is a photo of what is currently installed in your system. A clear image is usually enough for a supplier to make a match.

If you are speccing a new installation, be ready to discuss the following with your supplier:

1. Connection type
2. Process temperature range and pressure class
3. Material, or what the rest of the line is made from
4. Insertion depth and overall length
5. Whether this is a replacement or a new installation

Pyromation can configure virtually any combination of those specifications. Custom thermowells typically ship in about a week. Standard sizes are available from stock, including 4, 6, 9, 12, and 15 inches. 

Need help finding a thermowell for your process? Our team is here to help. Request a free consultation today with one of our product specialists.